Downgrade invalid packets message to warning.
Downgrade invalid packets message to warning.

Otherwise these show up in the logs while looking for errors, and they
are not really errors, just try jobs than weren't associated with an
issue.

R=rogerta@chromium.org
Review: https://codereview.appspot.com/6597064/

nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra